1. Field of the Invention
The present invention relates to a drive circuit for an electrostrictive actuator provided to a fuel injection valve in an internal combustion engine, i.e., gasoline or diesel engine.
2. Description of the Related Art
A conventional technique is known regarding the fuel injection valve control using an electrostrictive actuator. In general, the electrostrictive actuator is constituted by a plurality of pellet type piezoelectric elements laminated in the form of a cylinder. Usually, a type of ceramic mainly containing titanate zirconate lead is used as the piezoelectric element (so-called PZT element). This piezoelectric element has an important feature that makes it widely utilized for mechanical-electrical conversion or electrical-mechanical conversion. That is, when an electric field is applied in the longitudinal direction of the piezoelectric element, the thickness of the element is rapidly expanded by a small amount. Conversely, when the electric field is removed, the thickness of the element is rapidly contracted to the original state.
Accordingly, control of the fuel injection valve is performed by utilizing this characteristic of the piezoelectric element used as the electrostrictive actuator. In this case, it is necessary to provide a drive circuit for controlling the actuator by supplying a direct current (DC) voltage to the piezoelectric element. The drive circuit is constituted by at least one capacitor for charging, an inductor used at the time of charging, a switching element for control of the charge and discharge, and a diode for clamping. The drive circuit is, in general, provided to each actuator, and the actuator is provided to each fuel injection valve.
There are, however, some variations in the electrical characteristic of each of these parts, particularly, the inductors, and these variations cause fluctuations in the timing and stroke of the expansion and contraction of the piezoelectric element, which result in an imperfect control of a fuel injection.